1. Field of the Invention
The present invention relates to medical image processing techniques for generating a virtual endoscopic image from a three-dimensional medical image representing a tubular structure of a subject to be examined.
2. Description of the Related Art
In recent years, modalities such as MDCT (multidetector computed tomography) advanced, and high-quality three-dimensional (3D) medical images representing a subject to be examined became obtainable. As such 3D medical images are obtainable, medical image processing techniques for generating pseudo-3D images are being developed. An example of such techniques is a virtual endoscopic display method. In the virtual endoscopic display method, an image resembling an endoscopic image, which is obtained by imaging the inside of a tubular organ, is generated from plural two-dimensional tomographic images obtained by CT (computed tomography) (hereinafter, the image resembling a real endoscopic image is referred to as a “virtual endoscopic image”). Examination using virtual endoscopic display is beneficial, because it is less invasive than examination using a real endoscope. Further, examination using virtual endoscopic display can make it possible to observe the condition of the inside of a tubular structure behind an obstruction region, through which a real endoscope is not passable, and the like.
For example, regarding virtual endoscope examination using CT images of large intestines, results of many clinical trials have been reported, and the efficacies of virtual endoscope examination have been confirmed. The results of clinical trials include evaluation of polyp detection rates by virtual endoscopic display, and a comparison between a polyp detection rate by virtual endoscope examination and a polyp detection rate by real endoscope examination. Therefore, virtual endoscope examination on large intestines are expected to be adopted in more cases not only in pre-operation examination but also in screening.
As a technique related to virtual endoscopic display, a technique for sequentially displaying virtual endoscopic images viewed from plural viewpoints set on a center line in the lumen of the colon of a patient is known. In this technique, the center line of the lumen of the colon is set in advance as an observation path, and the virtual endoscopic images are sequentially displayed while a virtual endoscope is moved between the viewpoints at a given speed (for example, please refer to U.S. Pat. No. 6,272,366 (Patent Document 1)).
Meanwhile, instead of setting an observation path of a virtual endoscope in advance, a technique for dynamically setting an observation path of a virtual endoscope is known. Specifically, a weighted average of a direction from a present viewpoint (referred to as point P) to the center of a visual range of a virtual endoscope (the direction is referred to as visual range center direction V) and the direction of a longest ray among plural rays from the present viewpoint P to the wall of the lumen is obtained. The obtained weighted average direction is set as a new visual range center direction (the direction is referred to as visual range center direction V′). Further, a center position of a cross-section of the lumen in the vicinity of a position moved, along the present visual range center direction V, from the present viewpoint P by a predetermined distance is set as new viewpoint P′. At the new viewpoint P′, a new visual range center direction is set based on visual range center direction V′ at the viewpoint P′ and the direction of a longest ray from the viewpoint P′ in a similar manner to the aforementioned processing. Further, setting of new viewpoints based on visual range center direction V′ is repeated. Accordingly, an observation path is sequentially set (please refer to PCT Japanese Publication No. 2005-514086 (Patent Document 2)).
When the observation path of the virtual endoscope is set in advance as in Patent Document 1, a visual range center direction of the virtual endoscope at each viewpoint generally coincides with a local forward-movement direction of the virtual endoscope at the viewpoint, in other words, coincides with the direction of the center line of the tubular structure in the vicinity of the viewpoint. In such a case, at a curved region of the tubular structure, the visual range center direction differs from a direction in which the center of the tubular structure located further in the forward-movement direction of the virtual endoscope is observable along the center. Specifically, as schematically illustrated in FIG. 12, viewpoint VP1 on center line CL of tubular structure R-colon is located at a position in which the curvature of the tubular structure R-colon is small. Therefore, visual range center direction DC1 at the viewpoint VP1 substantially coincides with the global direction of the center line CL. Further, as illustrated in FIG. 13A, virtual endoscopic image I-VE1′ viewed from the viewpoint VP1 is generated in such a manner that the center of the tubular structure R-colon viewed from the viewpoint VP1 coincides with the center (indicated by the mark “+” in FIG. 13A) of image I-VE1′. However, at viewpoint VP2, the curvature of the tubular structure R-colon is large. Therefore, as illustrated in FIG. 13B, visual range center direction DC2 does not coincide with the global direction of the center line CL. Consequently, virtual endoscopic image I-VE2′ viewed from the viewpoint VP2 is generated in such a manner that the center of the tubular structure R-colon is shifted from the center (indicated by the mark “+” in FIG. 13B) of image I-VE2′ at positions away from the viewpoint VP2 by certain distance.
Meanwhile, in observation of a tubular structure by using a real endoscope, the visual range center direction of the real endoscope substantially coincides with a direction in which the center of the tubular structure located further in the forward-movement direction of the real endoscope is observable along the center. Therefore, a difference in the visual range center directions between the virtual endoscope and the real endoscope gives a sense of incongruity to users. To reduce such a sense of incongruity, the visual range center direction at a curved region may be changed by a manual operation of a mouse or the like by a user (observer). However, such an operation imposes an additional work on the user. Consequently, an observation time becomes long, and the efficiency of diagnosis becomes lower.